Electrophoresis has been known for more than 100 years and has been used in a variety of ways as a technique for the coating of metal articles. This technique has been employed for depositing metals, oxides, phosphors, rubber, paints, polymers and other materials using both aqueous and non-aqueous media. It has been used extensively in the commercial manufacture of rubber products from latex and in automotive painting.
In the ceramic manufacturing industry, the use of electrophoretic deposition has been rather limited. There are a few processes which have had substantial commercial value. One involves the electroforming of beta-alumina articles for use in high-energy sodium-sulfur and sodium-halogen batteries (see U.S. Pat. No. 3,946,751) and another involves the formation of thin continuous strips of clay, suitable for cutting into tiles or plates (see Chronberg U.S. Pat. Nos. 4,092,231 and 4,170,542). So far the use of the Chronberg process for manufacture of ceramic tile from clay suspensions has not been fully exploited.
Electrophoretic deposition is well suited to the manufacture of beta-alumina articles by the process described in U.S. Pat. Nos. 4,073,711 and 4,279,725 (General Electric). In that process an organic suspension of beta alumina particles is employed using amyl alcohol as the liquid media because of its dielectric properties. The particles are deposited on an electrically-charged mandrel to form thin-walled tubes with a diameter of about 1 centimeter and a wall thickness of about 1 millimeter as described in U.S. Pat. No. 4,279,725.
Electrophoresis has also been proposed as a method for speeding up the slip casting of clay earthenware or pottery as disclosed in U.S. Pat. Nos. 3,718,564 and 4,121,987. It has also been proposed for the manufacture of porcelain articles and porcelain-coated articles (see U.S. Pat. Nos. 3,484,357; 3,575,838 and 4,708,781).
The electrophoretic processes described above for use in forming of ceramic articles commonly employ aqueous suspensions containing additives, such as polyacrylic acid, triethylamine, ethanol, sodium carbonate, sodium hydroxide, sodium silicate, surface-active agents, deflocculants, etc.
In general, organic liquids are considered superior to water as a suspension medium for electrophoretic forming. The use of water-based suspensions causes a number of problems including gas evolution at the electrodes. This can cause bubbles to be trapped within the deposit. Special means have been proposed to minimize this bubble problem as by using a porous membrane and depositing the particles on the membrane as disclosed in U.S. Pat. Nos. 4,684,386 and 4,689,066. The bubble problem is less serious when using an organic suspension instead of an aqueous suspension.
The latter patents (U.S. Phillips Corporation) relate to the manufacture of thin-walled quartz-glass tubes for optical waveguides. U.S. Pat. No. 4,689,066 describes manufacture of a transparent glass tube with a diameter of 19 mm and a wall thickness of 1.2 mm from an homogenized anhydrous suspension of colloidal silica containing a quaternary ammonium compound. The organic media may be ethanol. The silica particles typically have a particle size of 15 to 100 nanometers (0.015 to 0.1 microns) with an average particle diameter of about 40 nanometers.
Electrophoretic deposition of coatings and the formation of thin-walled articles from colloidal silica can be feasible if the deposit is relatively thin. However, the deposited coating loses its conductance as the thickness of the deposit increases, thus retarding the rate of deposition. Because of this self-limiting characteristic, the buildup in the electrical resistance of the deposit can be a major problem when attempting to produce articles with substantial wall thickness.
There are a number of reasons why electrophoretic forming processes have so far achieved little commercial success. There are serious shortcomings in the fundamental understanding of the subject, and it is difficult to predict whether a given suspension will deposit electrophoretically in the desired manner. Laboratory testing has indicated that a large number of different powders can be deposited including barium and calcium carbonates, alumina, magnesia, zinc oxide, silica, titanium dioxide, indium oxide, tungsten carbide and various metals and phosphors.
It would be desirable to be able to predict from suitable parameters whether an electrophoretic deposition process will produce the desired results. The most commonly used parameters are zeta potential and electrophoretic mobility, but zeta potentials are difficult to measure or to interpret. Unfortunately there is no satisfactory theory that covers and explains all observations on electrophoretic deposition, and the subject is not well understood. Theoretical mathematical analysis has been attempted but is questionable because the equations used are based on assumptions regarding particle size and shape and theoretical models of doubtful validity (e.g., conveniently assuming that the charged particles are spherical when that is not true).
It appears that, because of lack of adequate information, misconceptions, prior failures, lack of experience or other reasons, the versatility of the advantages and potential advantages of electrophoretic deposition in the manufacture of improved glass and ceramic products were heretofore not appreciated prior to the present invention. In any event, research and development work in the field of electrophoretic deposition has been neglected, and the ceramic industry has relied on other forming processes.
In the field of investment casting where refractory shell molds are formed by the usual “lost-wax” process, it has been suggested that electrophoresis be employed during manufacture of the shell molds as disclosed in Szabo U.S. Pat. Nos. 3,850,733 and 3,882,010. In the proposed Szabo process the wax patterns are coated with graphite and dipped in an electrically-conductive coating suspension. The Szabo patents recognize that gas evolution at the anode or depository electrode creates major problems and that it is difficult to provide reliable results by electrophoretic deposition. These patents do not provide a reliable and commercially satisfactory process of substantial importance.
The problems associated with electrophoretic deposition are discussed in Norton U.S. Pat. No. 4,357,222 including the major problem of gas formation at the depository electrode (anode) from electrolysis of the slip liquid which causes serious flaws in the cast part. The Norton patent minimizes this bubble problem by providing a special non-conducting rubber mold having a relatively small anode at the bottom of the mold which forms only a small fraction of the forming surface of the mold and by moving one electrode relative to the other. A spherical casting of substantial size can be molded by filling the mold cavity with a suitable casting slip such as a suspension composed of about 86 percent by weight of silicon carbide, about 14 percent by weight of water and 0.1 percent by weight of sodium silicate. If the mold cavity is filled with a slip composed of about 50 percent water, about 50 percent elemental silicon and about 0.5 percent sodium silicate, a silicon casting is produced which can be converted to silicon nitride by standard nitriding.
The stability of the slip is less important in the non-conducting rubber mold of the Norton patent because the depository anode is at the bottom of the mold and attracts the particles in the same direction as gravity.
Unfortunately the Norton process has very limited utility and is unsuitable for formation of thick-walled articles, such as tanks, crucibles or other receptacles, where gravitational force can cause serious adverse effects.
For several decades, high-purity quartz glass articles used in the semiconductor industry have been produced by sintering porous slip-cast preforms as described, for example, in U.S. Pat. No. 4,072,489. It was known that methods other than slip casting, such as hot isostatic pressing and injection molding, could be used in making sintered quartz glass and that untested gel-casting or electrophoretic deposition methods were possible alternatives. However, serious research and development work on such alternative methods was not considered worthwhile or desirable. Prior to the present invention, there appeared to be no satisfactory substitute for slip casting and no reason to conclude or expect that problematical new methods, such as gel casting or electrophoretic deposition could become commercially successful or could provide a simple, safe, reliable and efficient process for commercial mass production of high-purity quartz glass products.